1. Field of the Invention
The present invention relates to a fuel cell system, and more specifically, to a fuel cell system which performs a temperature raising operation which increases the temperature of the fuel cell.
2. Description of the Related Art
Generally, in fuel cell systems, it is a known fact that as the fuel cell attains a predetermined temperature, the system shifts to a normal operation where constant power generation is possible. There has been a constant demand that the system should be able to shift to normal operation quickly, and there have been a number of proposals for a fuel cell system which performs a temperature raising operation which is aimed at a quick increase in the temperature of the fuel cell. For example, there are a fuel cell system which uses a heater to perform a temperature raising operation, and a fuel cell system which performs a temperature raising operation by decreasing the amount of the aqueous fuel solution circulating in the system. As another example, JP-A 61-269865 discloses a fuel cell system which performs a temperature raising operation by increasing the concentration of the aqueous fuel solution to a value higher than a value for normal operation. Also, JP-A 2006-286239 discloses a fuel cell system which makes an adjustment on the concentration of aqueous fuel solution and on the voltage of the fuel cell based on a difference between the temperature of the fuel cell and a target temperature, thereby achieving a stable power supply to electric appliances while performing a temperature raising operation.
In all of these techniques, the temperature raising operation is stopped once the temperature of the fuel cell has become not lower than a predetermined temperature. The fuel cell's cooling capability is influenced by the outside air temperature (ambient temperature), so if the outside air temperature is high, the temperature of the fuel cell continues to increase after the temperature raising operation has been stopped, and the temperature of the fuel cell becomes too high as compared to a predetermined target temperature. This poses a problem of accelerated deterioration of the fuel cell. In particular, fuel cell systems which use liquid fuel are apt to accelerated deterioration of the fuel cell since their thermal capacity is higher than those fuel cell systems which use gaseous fuel. In other words, once the temperature of the fuel cell systems becomes excessive, it will not decrease quickly.